Construction And Study On The CdS Quantum Dots-based Photoelectrochemical Aptasensor

The photoelectrochemical detection methods,which possess a number of useful capabilities,including high sensitivity,low cost,easy operation and rapid detection,are promising analytical tools for bioanalysis,clinical diagnostics,environmental monitoring and food safety.The different forms of excition?light?and detection?current?could reduce the undesired background signals,to allow a highly sensitive detection performance.Aptamers are single-stranded DNA or RNA oligonucleotides that can bind with high affinity and specificity to a wide range of target molecules such as metal ions,small organic molecules,drugs,proteins,and even whole cells.They are selected in vitro by process known as systematic evolution of ligands by exponential enrichment?SELEX?.Since their discovery in 1990,aptamers have attracted tremendous attentions as novel molecular recognition elements for sensing systems based on their advantages over traditional fluorescent probes.The OTA aptamer was firstly obtained and proposed by Cruz-aguado in2008.After that,several aptamer-based biosensors were demonstrated in the fields of biological and food analysis,while there are rarely related works in photoelectrochemical application areas.In this dissertation,we use OTA aptamers as biological recognition elements,CdS QDs based nanocomposites as photoactive species,to construct photoelectrochemical biosensorfor OTA detection.Firstly,TiO₂ nanoparticle is generally utilized in photoelectrochemical biosensor due to its nontoxicity,photostability,availability and photocatalysis.CdS QDs sensitized TiO₂ NPs are then synthesized and used to construct photoelectrochemical platform for OTA detection.In detail,we prepared a clean indium-tin oxide?ITO?electrode which was deposited with TiO2 NPs and sintered under certain temperature,then CdS QDs were adsorbed to obtain the CdS/TiO2/ITO electrode.After that,Ochratoxin A aptamers were bound on the surface of CdS/TiO₂/ITO electrode by EDC coupling reaction,the unbound sites were blocked with monoethanolamine.When various concentration of Ochratoxin A were added,the photocurrent would decrease correspondingly because of the steric hindrance.The decreased value is correlated with the OTA concentration.Under optimized condition,this photoelectrochemical sensing strategy shows a linear relationship between photocurrent variation and the logarithm of target OTA concentration in the range of 0.01-100 ng/mL and the detection limit of 0.001 ng/mL.The photoelectrochemical aptasensor exhibits high sensitivity and selectivity for Ochratoxin A analysis.Secondly,we have constructed a‘‘signal-on''photoelectrochemical OTA sensing strategy which is proposed on the basis of the resonance energy transfer between CdS quantum dots?QDs?and Au NPs.The CdS/ITO modified electrode was fabricated by alternately immersing the clean ITO slices into poly dimethyl dially ammonium chloride?PDDA?and CdS QDs for 10 min.Afterwards,probe DNA was modified on CdS/TiO₂/ITO electrode by EDC coupling reaction,the unbound sites were blocked with monoethanolamine.In the absence of Ochratoxin A,the AuNPs-aptamer acted as a signal quenching element could be introduced by hybridization with probe DNA,the photocurrent of CdS QDs was quenched for the energy transfer process.In the present of Ochratoxin A,the AuNPs-aptamer would combine with Ochratoxin A tightly,which could greatly hinder the hybridization between probe DNA and AuNPs-aptamer DNA due to the competitive occupation of binding sites and steric effect,leading to the recovery of photocurrent.Under optimized condition,this photoelectrochemical sensing strategy shows a linear relationship between photocurrent variation and the logarithm of target OTA concentration in the range of 0.05-20 ng/mL and the detection limit of 0.017 ng/mL.Lastly,the RGO/MoS₂ nanocomposites which were synthesized by a one-step solvothermal reaction were introduced into photoelectrochemical platform creatively,then CdS QDs were loaded on RGO/MoS₂ nanosheets to obtain RGO/MoS₂/CdS nanocomposites.The scanning electron microscope?SEM?and transmission electron microscope?TEM?were used to characterize these nanocomposites.Specifically,a certain amount of RGO/MoS₂/CdS nanocomposites were deposited on the surface of ITO slices.After modified with OTA aptamer by EDC coupling reaction,and the unbound sites were blocked with monoethanolamine,various concentration of Ochratoxin A were incubated with OTA aptamer modified electrods through biological recognition.Under optimal condition,this photoelectrochemical aptasensor was able to detect OTA in the linearrange from 0.001-5ng/mL with a detection limit of 0.3 pg/mL.